Review round 5: antialiased edge pixels still stacked where cone edges
coincided, and depended on draw order, because lighten compares colours
before alpha-compositing. Each cone's layer is now filled black before the
cone is drawn, so its coverage is baked into an opaque grey and lighten
takes a true maximum. The layer is clipped to the cone's own box instead of
the whole canvas.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>